Chrysocetus is a genus of extinct archaeocete ceteceans belonging to the Basilosauridae that is known from the Late Eocene of the eastern United States and western Africa.

In addition to being noted for its well preserved hip bones, Chrysocetus is unique amongst basilosaurids for possibly having only had a single set of teeth.

The alveoli (toothsockets) are separated by evenly spaced diastemas, which bear the marks left by the teeth of the mandible when the jaws were closed.

The jugal is missing both ends, but about halfway down its length the sides become noticeably flattened while the element expands in its height, thinning the further back it goes.

[1] The teeth of Chrysocetus are described as broadly resembling those of other basilosaurids, consisting of simple caniniform incisors and canines and more complex premolars and molars that feature numerous accessory cusps before and after the main apex.

The third lower premolar is thought to be the largest tooth in the jaw and possesses tall denticles that are well distinguished from the main apex.

The dorsal arch of the atlas is perforated by large foramina on either side and the axis is notably taller than all other neck vertebrae, owing to its robust neural spine.

[1] Both the cervicals of C. healyorum and those of C. fouadassii are similar in most aspects of their anatomy, but can be distinguished by the fact that the African species has significantly longer vertebral centra.

[3] The Chrysocetus holotype preserves 12 thoracic vertebrae, with Uhen and Gingerich arguing that many more were likely to have been present and were simply not recovered alongside the other material.

Finally, the caudal-most thoracic vertebra known from Chrysocetus has wide transverse processes and a singular articular facet for the ribs on either side.

[4] This is an important feature that clearly distinguishes Chrysocetus and other dorudontines from basilosaurines, which have highly elongated lumbar vertebrae.

[1] Overall the thoratic, lumbar and caudal vertebrae are noted for being relatively "normal" in their proportions, lacking the elongation seen in animals like Saghacetus and most prominent in Basilosaurus.

The elements of the sternal body are only fragments of the entire bone, but appear flattened towards the front with concave upper and lower surfaces.

The deltapectoral crest is the part of the humerus that extends the furthest cranially (i.e. in the direction of the skull), a point reached about two thirds down from the proximal end.

The humerus is one of the key elements that differentiates the North American species, C. healyorum, from the Moroccan C. fouadassii, as in the latter this bone is significantly longer.

In Chrysocetus the pubis is greatly elongated, the ilium highly reduced and the ischium mostly unknown due to breakage.

The dorsal side bears a protuberance not seen in other cetaceans or mammals but likely served as a point of attachment for musculature and ligament, which in life would have held the pelvic bones in place.

While these are clear signs of skeletal immaturity, meaning the animal was not fully grown at the time of its death, the teeth appear to contradict this notion.

This comparison has shown that, despite , despite having much more advanced tooth development, the holotype of Chrysocetus would fall in-between the two afforementioned archaeocete individuals in terms of age.

However, it has also been hypothesized that Chrysocetus could have been monophyodont, meaning that like in modern whales, they only ever developed a single set of teeth throughout their life.

[1] The initial phylogenetic tree featured in the type description of Chrysocetus was the result of statocladistics, meaning that morphological data was used in combination with the stratigraphy of the fossil remains.

In this tree basilosaurids were recovered as a paraphyletic group basal to modern whales, with each taxon gradually leading to crown cetaceans.

Basilosaurus was recovered as the basalmost member of this group, with Dorudontinae including the last common ancestor of Zygorhiza and Dorudon and all its descendents sans mysticetes and odontocetes.

In this study, Chrysocetus, on account of its advanced tooth development, was found to be the most derived dorudontine and possibly ancestral to modern whales.

[1] A similar conclusion was reached by Uhen in his 2009 chapter in the book "Encyclopedia of Marine Mammals", though the precise internal relationships of basilosaurids differ from those recovered previously.

[5] †Pakicetidae †Ambulocetidae †Remingtonocetidae †Protocetidae †Cynthiacetus †Basiloterus †Basilosaurus †Dorudon †Ancalecetus †Zygorhiza †Saghacetus †Chrysocetus Mysticeti Odontoceti The interpretation of Chrysocetus as a close relative of modern whales is contrasted by the phylogenetic tree produced by Antar and colleagues in 2023.

In their study, Chrysocetus appears as one of the basalmost dorudontines, clading together with Tutcetus from Africa and Ocucajea from South America.

A total of six early whales were discovered there, belonging to the families Protocetidae and Basilosauridae, all of which varying considerably in their size.